The signal component of an image signal generated by an information processing apparatus is sometimes contained in an unwanted electromagnetic wave unintentionally radiated from an information processing apparatus such as a computer, an external screen display device connected to the information processing apparatus, or a cable such as a signal cable or power cable connected to the information processing apparatus or screen display device. The unwanted electromagnetic wave is eavesdropped to reproduce original image information, illicitly acquiring various kinds of information contained in the image information.
As an information leakage prevention technique of preventing leakage of image information from an unwanted electromagnetic wave, there has been proposed an information leakage prevention apparatus which prevents reproduction of image information contained in an unwanted electromagnetic wave by generating a leakage prevention signal synchronized with an image signal from an information processing apparatus and superposing it on the unwanted electromagnetic wave (see, for example, patent literature 1).
FIG. 23 is a block diagram showing the arrangement of a conventional information leakage prevention apparatus. In an information leakage prevention apparatus 50, a basic clock generation unit 51 generates a basic clock signal BC having the dot clock frequency or its harmonic frequency of each dot data forming image information, based on a horizontal sync signal H or vertical sync signal V from an information processing apparatus 20. A modulation pattern generation unit 52 generates a fixed modulation pattern signal MP having each sign bit data synchronized with a pulse obtained by frequency-dividing the basic clock signal BC. A modulated clock generation unit 53 generates a modulated clock signal MC by modulating the basic clock signal BC in accordance with the modulation pattern signal MP.
FIG. 24 is a timing chart showing a clock modulation operation in the conventional information leakage prevention apparatus. In the modulation pattern generation unit 52, the repetition period of the modulation pattern signal MP is reset based on the horizontal sync signal H. Thus, predetermined pattern modulation is repetitively performed in the period of the horizontal sync signal H based on the basic clock signal BC.
An output amplification unit 54 amplifies the modulated clock signal MC generated by the modulated clock generation unit 53, generating a leakage prevention signal JC. The leakage prevention signal JC is output to the ground potential of the image signal of the information processing apparatus 20 or simultaneously to both the ground potential and image signal. Alternatively, the leakage prevention signal JC is transmitted from an antenna. As a result, the leakage prevention signal JC is superposed on an unwanted electromagnetic wave.
FIG. 25 exemplifies an image reproduced from an unwanted electromagnetic wave. FIG. 25 is merely a schematic view and does not faithfully express the reproduced image. In this example, a vertical stripe mask pattern (black part) corresponding to sign bit data of the modulation pattern signal MP is composited on original image information. The mask pattern degrades the visibility of the original image information reproduced from the unwanted electromagnetic wave, preventing information leakage.